MacBook Pro - Xbox 360 and MBP Internet

What I want to do is use my macbook pros ethernet port to plug an ethernet cord into and plug the other end of the cord into the back of my xbox 360 (theoreticaly it could be another computer) and have that other computer or my xbox 360 use the internet my macbook pro is getting wirelessly.

XBOX live is a very particular service. Even with a broadband router it requires support of UPNP (Universal Plug and Play) or an entry in the DMZ host slot to work properly. By work properly I mean showing up as highly available and not restricted, use your network troubleshooter to see your status on the XBOX 360.

I get the feeling you already have a broadband router as your mac is getting on the internet wirelessly in which case it sounds like you need the XBOX 360 Wireless adapter. I don't think you are going to be able to McGuyver yourself a wired connection the way you are describing.